Optimal number vertices in a game
If this is the terrain we're talking about, 120k is probably a good number, seeing as the base reqs for a computer are 1,000,000 verts (without any other things, textures, filters, light). That mean most computers can handle vertex counts of up to 1mil. 

The only problem I can see is in the fact that this is a terrain, and not a character, you said only 1-2k verts would be seen at a time, but that doesn't matter if the terrain is one object; Blender will still render the entire object even if only 1 vertex is visible.

So what to fix then? Generally, terrains don't need this amount of detail (right now I'm making one, several acres, and its pushing 30k), so to start with, you may wan to break it down into more base elements to free up resources for other rendering jobs. If its a city landscape, separate the buildings into their own objects, and get some LOD going for props. You may also want to break the landscape into more manageable chunks an area that would normally be blocked by a hill could be separated into its own object so that it only need to be rendered when called for. You could also use occlude objects. these basically ensure that any object that is hidden behind another won't be rendered. It seems intuitive, but really, blender renders anything inside your cameras view arc and clipping range whether or not it's behind another object.

There is a whole science behind game optimization, and youTube will probably be your best resource.